Industrial robot having a function for controlling a current of a motor for driving

ABSTRACT

Disclosed is an industrial robot having a function for controlling a current of a motor for driving, which comprises a circuit (41) for summing a plurality of input signals to which input signals of speed command, speed feedback, current command and current feedback are supplied, a switching circuit (23) for selectively switching the input of speed command to the inputs of current command and current feedback by a switching command signal and an operational amplifier (38) to which the switched signal is supplied. In this industrial robot, the position control is switched over to the current control by a switching command signal from a robot control device (22), and by the position control, an article to be held is shifted to a predetermined position and by the current control, driving electric motors (Mr, Mθ, Mz) are operated under a certain output torque to press the article to be held against other article under a certain torque, whereby the reliability of the operation of the industrial robot can be increased.

DESCRIPTION

1. Technical Field

The present invention relates to an industrial robot having a functionfor controlling a current of a motor for driving.

2. Background Art

A direct current electric motor has heretofore been used for driving theshafts of an industrial robot, and a control circuit, as shown in FIG.1, has been used for controlling the driving of the direct currentelectric motor.

The control circuit shown in FIG. 1, is operated as follows.

A position command input 11 is given, and in an adder 12, a differencebetween this position command input 11 and a position feedback signal 18from a pulse coder 17 connected to an electric motor 16 is calculatedand this difference is set at an error register 13. The set value isconverted to an analog value by a digital-analog converter (DAconverter) 19 and is supplied as a speed command signal to one input ofa speed-controlling amplifier circuit 15. The position feedback signal18 from the pulse coder 17 is converted to a voltage signal by afrequency-voltage converter (VF converter) 14 and this voltage signal issupplied as a speed feedback signal to the other output of thespeed-controlling amplifier circuit 15. Driving of the electric motor 16is controlled by the output of the speed-controlling amplifier circuit15.

In an industrial robot comprising the above-mentioned control circuit,however, it is impossible to press an article to be held against anotherarticle under a certain output torque. For example, in the case where anarticle to be held is held only by the position control in a desirablefixed state on a chuck of a machine tool, for example, a lathe, it isdifficult to hold the article on the chuck in a desirable fixed state.Accordingly, in an industrial robot having the above-mentioned circuit,though position control characteristics are satisfactory, the operationof pressing an article to be held against another article under acertain output torque cannot be performed.

The present invention proposes an industrial robot in which this problemof the conventional technique is solved.

DISCLOSURE OF THE INVENTION

It is a primary object of the present invention to provide an industrialrobot, in which, in order to solve the above problem of the conventionaltechnique, control of the position is performed by a position commandand when an article to be held arrives at a predetermined position, theposition control is switched over to the current control by a switchingcommand signal, whereby it is made possible to press the article to beheld against another article and the reliability of the operation of theindustrial robot can be increased.

In accordance with one aspect of the present invention, there isprovided an industrial robot having a function for controlling a currentof a motor for driving, which comprises a circuit for summing aplurality of input signals to which input signals of speed command,speed feedback, current command and current feedback are supplied, aswitching circuit for selectively switching the input of speed commandto the inputs of current command and current feedback by a switchingcommand signal and an operational amplifier to which said switchedsignal is supplied.

In accordance with another aspect of the present invention, there isprovided an industrial robot having a function for controlling a currentof a motor for driving, which comprises a robot control device, aswitching circuit including a first switch which receives an output ofthe robot control device through a register and a digital-analogconverter, is inserted in a circuit for input of the speed command andis closed at the position control while it is opened at the currentcontrol and a second switch which is inserted in a circuit for input ofthe current command and input of the current feedback and is opened atthe position control while it is closed at the current control, acircuit for summing a plurality of input signals which receives theoutput of the switching circuit and to which inputs of speed command,speed feedback, current command and current feedback are supplied, anoperational amplifier for receiving the output of the circuit forsumming a plurality of input signals, and a driving electric motor forreceiving the output of the operational amplifier.

BRIEF EXPLANATION OF THE DRAWINGS

FIG. 1 is a circuit diagram illustrating a driving electricmotor-controlling circuit used for the conventional industrial robot.

FIG. 2 is a diagram illustrating one embodiment of the industrial robothaving a function for controlling a driving electric motor according tothe present invention.

FIG. 3 is a characteristic diagram illustrating the relation between aspeed command signal and an electric current of an electric motorobserved when the position control is carried out in the circuit shownin FIG. 2.

FIG. 4 is a characteristic diagram illustrating the relation between acurrent command signal and an electric current of an electric motorobserved when the current control is carried out in the circuit shown inFIG. 2.

BEST MODE FOR CARRYING OUT THE INVENTION

One embodiment of the industrial robot having a function for controllinga current of a motor for driving according to the present invention isillustrated in FIG. 2. One control circuit 21 is disposed for each ofthe three direction components r, θ and z determining the position ofthe robot, and the control circuits 21 have the same function. An input32 of a position command signal is supplied to an adder 24, and thedifference between this input 32 and a position feedback signal 34 froma pulse coder connected to an electric motor for driving the robot inany of the above three directions, for example, an electric motor Mz, isobtained from the output of the adder 24 and the output signal of theadder 24 is set at an error register 25. The output of the errorregister 25 is connected to a first switch 35 of a switching circuit 23through a DA converter 27. An input 33 of a current command signal isset at a current value register 26 and is connected to one contact of asecond switch, that is, an interlocking switch 36, of the switchingcircuit 23 through a DA converter 28. A position feedback signal 34 issupplied to a frequency-voltage converter 29 and converted to a voltagesignal. The voltage signal is supplied to an input of an operationalamplifier 38 through a circuit 41 for summing a plurality input signals.A current feedback signal 37 from a resistor 31 for detecting anelectric current of the driving electric motor is supplied to the othercontact of the interlocking switch 36. The speed command signal whichhas passed through the switch 35 is supplied to the input of theoperational amplifier 38 through a first input of the circuit 41 forsumming a plurality of input signals. A speed feedback signal 40 fromthe frequency-voltage converter 29 is supplied to the input of theoperational amplifier 38 through a second input of the circuit 41 forsumming a plurality of input signals. The current command signal andcurrent feedback signal which have passed through the interlockingswitch 36 are supplied to the operational amplifier 38 through third andfourth inputs of the circuit 41 for summing a plurality input signals.An output of a speed command amplifier circuit 30 is connected to thedriving electric motor through the resistor 31 for detecting an electriccurrent of the electric motor.

The industrial robot shown in FIG. 2 is operated as follows.

Switching between the position control and the current control isperformed by the switching command signal 39 from the robot controldevice 22. This switching command signal 39 is supplied to the switchingcircuit 23 to control the opening and closing of the switch 35 and theinterlocking switch 36. When the position control is carried out, theswitch 35 is closed by the switching command while the interlockingswitch 36 is opened, whereby the speed command signal and speed feedbacksignal are supplied to the input of the operational amplifier 38 andcontrol is made so that the article to be held is shifted to a desirableposition according to the position command signal 32 from the robotcontrol device 22. When the position control is changed over to thecurrent control, the switch 35 is opened by the switching command signal39 while the interlocking switch 36 is closed, whereby the currentcommand signal, current feedback signal and speed feedback signal aresupplied to the input of the operational amplifier 38. The drivingelectric motor is controlled by an output signal of the current valueregister 26 set by the current command signal 33 from the robot controldevice 22. In this case, not only the current feedback signal, but alsothe speed feedback signal, is put in the operational amplifier 38.Accordingly, the driving electric motor is rotated at a stable speed,and when the movement of the article is blocked by a stationary wall,such as a chuck end face 43 of a lathe 42 and the article is stopped,the speed feedback signal becomes equal to zero, and, hence, the currentvalue of the driving electric motor becomes equal to the set currentcommand value. Namely, a certain output torque is obtained in thedriving electric motor. The change of the position caused by rotation ofthe driving electric motor by the current control is stored in the errorregister 25 in advance so that the position information is retained.

The relation between the speed command signal Vv and the electriccurrent A of the direct current electric motor observed when theposition control is carried out is illustrated in a characteristicdiagram of FIG. 3. From the characteristic diagram of FIG. 3, it is seenthat by a very small speed command signal VR(a) (the order ofmillivolt), the electric current of the direct current electric motor iselevated substantially to a level of the rated current Ir. Morespecifically, when the position is deviated from the predeterminedposition, the direct current electric motor is rotated at a rated outputto move the article to the vicinity of the predetermined positionpromptly, and when the article is brought close to the predeterminedposition, the speed of the electric motor is abruptly reduced and themotor is stopped when the article is set at the predetermined position.In this case, a very large output torque is produced even for a slightdeviation of the position.

The relation between the current command signal Vc and the electriccurrent A of the direct current electric motor observed when the currentcontrol is carried out is illustrated in a characteristic diagram ofFIG. 4. From FIG. 4, it is seen that the electric current A of thedirect current electric motor has a relatively small gradient ascompared with the current command signal Vc. In this case, by causingthe current feedback signal and speed feedback signal to co-operate witheach other, the current control can be performed stably and the directcurrent electric motor can be driven under a certain output torque. InFIG. 4, Ir represents the rated current and VR(b) is several hundredmillivolts.

We claim:
 1. An industrial robot having a function for controlling acurrent of a motor for driving, having a robot control device, threeelectric motor control circuits for controlling the positioning of saidrobot in three directions, and three electric motors driven through saidelectric motor control circuits, respectively, each of said electricmotor control circuits comprising:an adder means for receiving aposition command signal from said robot control device; a circuit meansfor summing a plurality of input signals and for receiving the output ofsaid adder; and an amplifier circuit means for amplifying the output ofsaid summing circuit means, a position feedback signal from a pulsecoder linked to said electric motor being fed back to said adder meansand said summing circuit means, wherein each of said electric motorcontrol circuits comprises a switching circuit between said adder meansand said summing circuit means, said switching circuit receiving theoutput of said adder means, a current command signal from said robotcontrol device, a current feedback signal corresponding to the drivecurrent of said electric motor, and a switching command signal from saidrobot control device, said switching circuit being switched so that theoutput of said adder means is supplied to said summing circuit means atthe positioning control, the current command signal and the currentfeedback signal being then supplied to said summing circuit means atcurrent control.
 2. The industrial robot of claim 1, wherein saidswitching circuit comprises a unipolar switch and a bipolar interlockingswitch.